Fibre optic distributed acoustic sensing (DAS) is a known type of sensing where an optical fibre is deployed as a sensing fibre and repeatedly interrogated with electromagnetic radiation to provide sensing of acoustic activity along its length. Typically one or more input pulses of radiation are launched into the optical fibre. By analysing the radiation backscattered from within the fibre, the fibre can effectively be divided into a plurality of discrete sensing portions which may be (but do not have to be) contiguous. Within each discrete sensing portion mechanical disturbances of the fibre, for instance, strains due to incident acoustic waves, cause a variation in the properties of the radiation which is backscattered from that portion. This variation can be detected and analysed and used to give a measure of the disturbance of the fibre at that sensing portion. Thus the DAS sensor effectively acts as a linear sensing array of acoustic sensing portions of optical fibre. The length of the sensing portions of fibre are determined by the characteristics of the interrogating radiation and the processing applied to the backscatter signals but typically sensing portions of the order of a few meters to a few tens of meters or so may be used.
DAS has been used in a number of applications such as perimeter security and monitoring of linear assets such as pipelines. One particular application where it has been proposed that DAS sensors may be employed is in monitoring movement of traffic on a transport network, for example monitoring trains moving on a rail network.
For traffic monitoring sensing fibres can be deployed to run generally along the path of at least parts of a transport network, for instance with sensing fibre(s) being deployed to run generally along the path of one or more rail tracks or a rail network or roads of a road network. Movement of a vehicle on the transport network, e.g. a train on a train track, adjacent a DAS sensing fibre will generate acoustic signals that can be used to track the vehicle as it moves, providing real time positional information to a resolution of a few tens of meters continuously along the entire length of the monitored section. DAS has several advantages for such traffic monitoring. DAS can be applied to provide many sensing channels over a long length of fibre, for example DAS can be applied on fibre lengths of up to 40 km or more with contiguous sensing channels of the order of 10 m long. Thus a long stretch of the transport network can be monitored using a single DAS sensor. For lengths of more that 40 km or so several DAS sensors units can be deployed at various intervals to provide continuous monitoring of any desired length of the transport network.
The sensing fibre may be standard telecoms fibre and thus is relatively cheap. The fibre may be simply buried alongside the transport networks, e.g. along the sides or underneath tracks or roads in a narrow channel and is relatively easy to install. The optical fibre can be encased in a protective casing and can survive for a long time with no maintenance. Thus installation and maintenance costs are low. In many transport networks there may already be optic fibre deployed along at least the major routes and such existing communications infrastructure may comprise redundant optical fibres that can be used for DAS.
The optical fibre is interrogated by optical pulses generated by the interrogator unit and thus power is only needed for the interrogator units.
In a transport network setting a DAS system thus provides the ability to achieve both a desired spatial resolution and scope of coverage that would be very difficult and costly to achieve using other sensing technologies and which allows for real-time monitoring and/or control of the transport network.
As mentioned above to provide coverage over a large area of interest, such as with a typical transport network, it may be necessary to have a plurality of sensing fibres covering different parts of the area of interest, each sensing fibre being interrogated by a suitable DAS interrogator unit. This may thus involve a plurality of DAS interrogator units being deployed in the area to be monitored, at least some of which may be remote from one another.